The present invention relates to low-emissivity glazing units, i.e. glazing units that have the ability to reflect infrared radiations emitted, for example, by the interior of residences, and therefore restricting heat losses.
The demand for such glazing units is often associated with the requirement to have as high a light transmission as possible. The two requirements of low emissivity and high transmission normally result in opposing solutions in terms of structure. It is necessary to make compromises that are difficult to achieve.
Added to these requirements are those of having glazing units that are as neutral in colour as possible, in particular in reflection. Finally, the production must remain as economical as possible.
To obtain these results, the most usual practice is to have systems of thin layers available having one or several layers that are able to reflect infrared radiations. Systems of this type generally comprise one or more metal layers, in particular silver layers several nanometers thick. The layers must be sufficiently thin so that they do not reduce the visible light transmission too significantly. The thickness must also be sufficient to hinder the transmission of infrared rays, wherein the thickness directly determines the fraction of those effectively reflected.
The systems applied to the glazing units must at the same time meet other conditions. Firstly, it is necessary to ensure that the systems resist any chemical or mechanical attacks they may be exposed to. The metal layers are usually deposited onto the glass substrate using magnetic field-assisted sputtering-type vacuum deposition, commonly known as “magnetron sputtering”. The layers obtained by these techniques have the advantage of a high uniformity of composition, thickness and surface condition. However, they are very fragile and must be protected by additional layers. Transparent dielectric layers of metal oxides and/or nitrides and/or oxynitrides or also mixtures thereof that provide the required resistance are most traditionally used.
At the same time, the metal layers must also be protected from any possible diffusion from the substrate that would unfavourably modify the properties of the reflective metal layer. The nature of dielectric layers located between the substrate and the metal layer is often the same as that of layers located above this same metal layer. These concern metal oxides and/or nitrides and/or oxynitrides.
Traditionally, the sequence of layers is configured as follows:
glass/dielectric I/metal/dielectric II
wherein each of the dielectrics I and II most frequently comprise several layers of a different nature.
The most frequently used dielectrics are in particular ZnO, TiO2, SnO2, Si3N4 . . . and their alloys. These dielectric layers provide various optical properties and are also distinguished by their conditions of industrial production.
The most customary structures additionally integrate a special layer between the metal and the outer dielectric, said layer having the function of protecting the metal in particular during deposition of the layer of this dielectric.
In fact, the formation of this dielectric is most often conducted in a so-called “reactive” manner. In this mode of production, the dielectric (oxide or nitride) is formed at the same time as the deposition by metallic vapour emitted by bombardment of a metal cathode with the atmosphere at very low pressure, in which this deposition occurs: an oxygen atmosphere or a gaseous mixture containing oxygen in the case of an oxide. In these conditions, the metal layer deposited is in contact with this atmosphere and can deteriorate in particular because of the high reactivity of the plasma.
To protect against this deterioration, it is customary to arrange a so-called “barrier” or “sacrificial” layer on the infrared reflective metal layer. This concerns a layer of very low thickness, whose function is to prevent any possible deterioration of the infrared reflective metal layer in particular when the upper layers are being deposited.
The barrier layer is carefully selected both for its nature and for its thickness. To prevent it from substantially reducing the light transmission, it is important to ensure that the barrier layer is as thin as possible, while also being highly transparent at the end of the production process of the multilayer stack.
Traditional systems therefore have the following layer sequence:
glass/dielectric I/metal/barrier/dielectric II
The metal layers, as indicated above, are those that selectively reflect infrared rays and therefore determine the emissivity of the assembly. While different metals are designated in specialist literature, practically all existing products use layers based on silver as reflective metal, and the silver can contain “doping” elements. In fact, it represents the best compromise in terms of infrared reflection and transparency to radiations in visible wavelengths and neutrality in colour in transmission and reflection. To simplify matters, the metal layer will be systematically presented as a silver layer hereafter.
Different means have been proposed to ensure that these silver layers attain the best performance rates. The instruction of the US publication 5 110 662 belonging to the applicant can be noted in particular, wherein the decisive influence of a ZnOy layer arranged directly below the silver layer and having a well defined thickness is demonstrated. It must be emphasised that variants of this idea have been used in several subsequent patents or patent applications such as WO 99/00528.
Various hypotheses have been projected to explain the mechanism that causes this ZnOy layer to improve the emissivity and conductivity properties under certain conditions. Some of these hypotheses concern, for example, the silver “hooking” onto the dielectric layer, while others consider that the presence of ZnOy benefits the crystallisation of the silver in systems resulting in fewer particle boundaries etc.
The conductivity and consequently the emissivity of the silver layers deposited in industrial conditions have been appreciably improved over time without reaching the ideal values of metallic silver. The choice is naturally to use layers that have the best conductivity and therefore the best emissivity possible. In the absence of having perfect silver layers available, it would appear that an additional improvement in emissivity could only be obtained by increasing the thickness of the silver layer.
It is well known that emissivity decreases when the thickness of the silver increases. Nevertheless, the consequences of this increase in the thickness of the silver are not all favourable. While the light transmission is affected relatively little, within the usual limits of variations in thickness of the silver layer, the main difficulty lies in the significant deterioration in resulting colorations in particular in reflection. The glazings in question tend to lose their neutrality.
For this reason in particular, the inventors have endeavoured to further improve the layer systems to obtain glazings, wherein the emissivity is also reduced while preserving the light transmission and an acceptable colour as far as possible.